In recent years, an increasing concern of global energy usage and its impact on the environment, in particular global warming, has resulted in extensive research into novel technologies of generating electrical power. Thermoelectric power generators have emerged as a promising alternative green technology due to their distinct advantages. In general, thermoelectric power generators offer a potential application in the direct conversion of waste-heat energy into electrical power irrespective of the cost of the thermal energy input.
A thermoelectric device can be used as a thermoelectric power generator or a thermoelectric cooler. Applications of these devices range from, for example, electronic thermal management and solid state refrigeration to power generation from waste heat sources. A thermoelectric generator is a solid state device that provides direct energy conversion from thermal energy (heat) due to a temperature gradient into electrical energy based on a so-called “Seebeck effect.” The thermoelectric power cycle, with charge carriers (electrons) serving as the working fluid, follows the fundamental laws of thermodynamics and intimately resembles the power cycle of a conventional heat engine. Thermoelectric power generators offer several distinct advantages over other technologies including, for example, high reliability, small footprint but with potential scaling to meet large area applications, lightweight, flexibility, and non-position dependency.
A major challenge of thermoelectric devices is their relatively low conversion efficiency, which is typically -5%. This has been a major cause in restricting their use in electrical power generation and thermal management to specialized fields where space and reliability are a premium.
The figure-of-merit (ZT) of a thermoelectric material is a dimensionless unit that is used to compare the efficiencies of various materials. ZT is determined by three physical parameters: the thermopower a (also known as a Seebeck coefficient), electrical conductivity a, and thermal conductivity k=ke+kph, where the ke and kph are thermal conductivities of electrons and phonons, respectively; and absolute temperature T:
  ZT  =                              α          2                ⁢        σ                    (                              k            e                    +                      k                          p              ⁢                                                          ⁢              h                                      )              ⁢          T      .      
Maximum ZT in bulk thermoelectric materials is governed by the intrinsic properties of the material system. Most candidates require low thermal conductivity as the driving force for enhanced ZT because of the inverse relationship between the Seebeck coefficient and electrical conductivity. This interdependence and coupling between the Seebeck coefficient and the electrical conductivity makes it difficult to increase ZT>1, despite nearly five decades of research. Increasing this value to 2.0 or higher will disrupt existing technologies and will ultimately enable more widespread use of thermoelectric systems.
In L. D. Hicks and M. S. Dresselhaus, Effect of quantum-well structures on the thermoelectric figure of merit, Phys. Rev. B, Vol. 47, No. 19, 12727-12731 (May 15, 1993), Hicks and Dresselhaus pioneered the concept of quantum confined structures that could significantly increase ZT by independently optimizing the Seebeck coefficient and electrical conductivity. Since then, numerous research groups have adopted nano-structured approaches to increase ZT and have ultimately determined that the enhancement resulted from reduced thermal conductivity from phonon scattering at the interfaces. In J. P. Heremans, V. Jovovic, E. S. Toberer, A. Saramat, K. Kurosaki, A. Charoenphakdee, S. Yamanaka, and G. J. Snyder, Enhancement of Thermoelectric Efficiency in PbTe by Distortion of the Electronic Density of States, Science, Vol. 321, 554-557 (Jul. 25, 2008), Heremans showed a significant improvement in the Seebeck coefficient by distortion of the electronic density of states through the use of impurity levels.
An alternative approach recently investigated to enhance thermoelectric performance using nano-structured materials is hot carrier transport via thermionic emission. The design criteria require a potential barrier of several kBT (where k is Boltzman constant, and T is temperature) to selectively transport high-energy “hot” carriers. The distribution of hot carriers at energy greater than the barrier height with respect to the Fermi level defines the Seebeck coefficient and the integral of this distribution defines the conductivity. Enhancement of the Seebeck coefficient has been observed by hot carrier transport for several material systems. This enhancement will be offset to some extent by a decrease in the electrical conductivity since fewer carriers are participating in transport. Thus, the overall impact on ZT will be highly dependent on the material system.
There is a need for a nano-structured thermoelectric material formed in a material system that maximizes, or at least significantly improves, ZT through hot carrier transport via thermonic emission.